The useful life of A.C. series universal motors and small D.C. motors that typically find application in appliances., tools and the like, as well as in many industrial contexts, is usually limited to the length of the carbon brush, its rate of wear, and--in the case of replaceable brushes--the number of times the brushes can be replaced before the commutator begins to wear severely.
It is understood that the rate or wear of the brush is a function of the load, the speed of the motor, and most importantly, the spring pressure that is applied to the brush to keep it, in bearing contact with the commutator. The curve of brush wear plotted against spring pressure, however, forms a parabola. Accordingly, it will be appreciated that with too much spring pressure the mechanical wear will become excessive, an improper film is formed on the commutator, and the brush life falls markedly. On the other hand, with too little pressure applied, the electrical arcing due to the high contact resistance and the loss of electrical contact greatly reduce the possible life of the brush and the contact surface.
The typical motor assembly rigging involves a helical spring bearing on the carbon brush, the two elements being combined in a box-like holder such that the brush is urged against the commutator. Although this design is used universally, it has several limitations.
The pressure produced by a helical spring is a function of its compression or extension. Therefore, when the brush assembly is brand new, and the brushes are at maximum length, the spring is at its fullest compression and the pressure therefore at its highest; at the end of the brush life, the spring extension is at its greatest and the pressure now typically is below the ideal. Therefore, depending on the spring rate, only a portion of the brush wear is in the ideal spring pressure range.
In addition To the force deflection curve, the helical spring also has a finite collapsed length. Accordingly, since the spring is generally located behind the brush in accordance with the usual way of enclosing it in the brush box, the space that it requires dictates that a shorter brush be used.
A long-life brush design using a constant force spring that is essentially wound like a clock spring and is set to unwind in such a direction as to hold the brush against the commutator has been used to overcome some of the problems mentioned above with the helical springs. By using a constant force spring, the ideal pressure range on the brush can be obtained, thereby obtaining minimum wear on the brush from this aspect.
It has been known to utilize an electrically conductive helical coil spring to provide electrical connection between the one end of a brush and a source of electricity. An example of such a device may be found in U. S. Pat. No, 3,376,444. An example of a brush holder utilizing a constant force spring through which current is flowed to the outer end of the brush may be found in U.S. Pat. No. 3,387,156 with particular reference being made to the embodiment illustrated in FIG. 6 thereof. An example of a brush assembly utilizing a constant force twin coil spring may be found in U.S. Pat. No. 2,695,968, and particularly the embodiment illustrated in FIG. 6 thereof.
The problems encountered with the constant force springs for brush holder assemblies include the difficulty in holding the brush and spring in place and the difficulty in replacing the brush when it is worn.